THE VEGETATION OF A DESERT MOUNTAIN RANGE AS 

 CONDITIONED BY CLIMATIC FACTORS. 



INTRODUCTION. 



The southern half of the state of Arizona may be briefly character- 

 ized as a relatively level plain studded with numerous hills and moun- 

 tains. The plain rises from elevations of a few hundred feet along the 

 Colorado River to as much as 4,500 and 5,000 feet near the New Mexi- 

 can boundary. The lower elevations follow the Gila, Salt, San Pedro, 

 and other rivers, while the higher plains surround the loftier mountains 

 of the southeastern portion of the State. Between the Colorado River 

 and Tucson there are no mountains of commanding elevation, and the 

 area occupied by the scattered volcanic peaks and ranges is not more 

 than one-tenth of the total area of the region. To the eastward of 

 Tucson, however, a much greater percentage of the total area is occu- 

 pied by mountain ranges, a score of which reach elevations of over 8,000 

 feet. The general topographic configuration of the region has remained 

 unchanged throughout a long period of geological time, and the moun- 

 tains and hills have been subjected to prolonged erosion, the products 

 of which have served to build up the shelving plains which form the 

 intervening valleys. 



Those portions of southern Arizona which lie below 4,000 feet are 

 covered with a low, open, desert vegetation, while the plains and valleys 

 of higher eleyation support a loose carpet of perennial grasses and 

 ephemeral herbs, together with a sparse representation of succulent and 

 semi-succulent types of plants. The higher mountain ranges exhibit 

 a graduated sequence of vegetation from that of the desert valleys, 

 through a scrub of evergreen oaks to forests of pine, spruce, and fir. 

 The bodies of mesophilous vegetation which occupy these isolated 

 mountain summits, and the stages which connect them with the vege- 

 tation of the desert, present innumerable phenomena of the greatest 

 interest to both physiological and floristic plant geography, and form 

 a most fruitful field of investigation. 



The Santa Catalina Mountains are one of the most westerly of the 

 high ranges of southeastern Arizona, and rise from an approximate 

 basal elevation of 3,000 feet to a height of 9,150 feet. With respect to 

 their vegetation these mountains are typical of a large number, not 

 only in Arizona but in southern New Mexico and northern Mexico as 

 well. Their location within 20 miles of Tucson and their ready acces- 

 sibility from the Desert Laboratory have given opportunity for a study 

 of the distribution of their vegetation and for a measurement of some 

 of the physical factors upon which the existence and activities of the 

 vegetation depend. It is the purpose of the present paper to give a 

 brief description of the vegetistic features of the various altitudes and 



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